In recent years, chemical mechanical polishing (CMP) has emerged as a viable and important process in integrated circuit fabrication. One of its most important applications is in planarization of interlevel dielectric layers (ILD's) in multilevel metallization structures, to improve lithographic resolution and to avoid metal step coverage problems. Another application of CMP is in the via fill process known as the Damascene process, wherein a metallic via plug is created by depositing metal into the via and onto the ILD surface, and the metal is thereafter polished off the surface leaving the via plug. A similar process, which forms the via plugs and the next level of interconnect metal in a single metal deposition, is known as the dual Damascene process. In this process, a second dielectric etch immediately follows the via etch, and forms a patterned recess in the ILD. Next, the metal is deposited and the surface excess is polished off using CMP, leaving the next level metal lines remaining in the ILD recesses. New applications for CMP in integrated circuit processing are still being identified.
CMP is generally performed by a polishing pad mounted on a hard platen, wherein the platen typically rotates during polishing. Wafers to be polished are positioned in a wafer carrier and inverted onto the polishing pad. A soft liner called the carrier film or carrier pad provides an interface between the hard wafer carrier and the wafer. The carrier film enables substantially uniform pressure to be applied to the wafer. Additionally, the high friction between the carrier film and the wafer generally results in no wafer rotation with respect to the carrier, although the wafer is not fixedly attached to the carrier. The carrier and wafer typically rotate about the carrier axis over the rotating platen, generally in the same direction, as illustrated in FIG. 2.
A polishing slurry is dispensed over the polishing pad to simultaneously provide mechanical abrasion and chemical interaction with the material to be polished, the mechanical and chemical components together causing surface material to be polished off. By way of example, a polishing slurry for polishing a silicon dioxide ILD film may comprise an SiO.sub.2 colloid in H.sub.2 O, pH adjusted with KOH to have a pH value of 10--11.
Polishing rate, also termed removal rate, is a function of the applied pressure between wafer and polishing pad, as well as of the relative velocity between the two. It can be approximately modelled according to Preston's equation as
dT/dt=-(KZa)ds/dt, where
dT/dt=rate of change of thickness of the wafer PA1 L/a=applied pressure over area a (L is the load force on area a) PA1 ds/dt=relative velocity between wafer and pad PA1 K=Preston's coefficient, a proportionality constant which is not a function of pressure or relative velocity.
In the case of CMP, Preston's coefficient will account for the chemical component of the polishing process. In addition, the amount of slurry at the site in question may be a factor in determining the proportionality constant.
An ideal CMP process would provide uniform removal rate across each wafer, and constant removal rate as a function of polishing time. In reality, within-wafer-non-uniformity (WIWNU) is a widespread concern in virtually every application of CMP, particularly as wafer diameters increase. WIWNU of CMP removal rate affects the uniformity of dielectric or metal layer thickness, and can also cause dielectric or metal dishing. These effects degrade device functionality, reliability, manufacturability, and yield. Consequently, an important goal in CMP is to reduce WIWNU. WIWNU in metal and oxide CMP is greatly affected by geometrical and physical effects such as carrier pressure non-uniformity and slurry dispense non-uniformity, by way of example.
Dielectric CMP is generally performed using a polishing pad made of polyurethane, by way of example. The physical and chemical properties of this type of pad change as polishing proceeds, and these changes can contribute to WIWNU. It is known that the continuous compression experienced by the polishing pad causes gradual decrease in its elastic modulus, G. Since the pressure P exerted at a point on the pad is approximately expressed by the elastic equation P=G.epsilon., where .epsilon. is the strain of the pad at that point, changes in G can affect the local pressure and consequently the local removal rate. As is illustrated in FIG. 2, the central region 26 of the polishing pad, which passes under the central wafer portions, is compressed for a greater percentage of the polishing time than are the inner and outer pad regions 24 and 28, which only contact the wafer edges. Consequently, the effective elastic modulus decreases faster in the central pad region than at the inner and outer regions, and the polishing rate after initial break in is therefore progressively lower at the wafer centers than at the wafer edges.
Another characteristic of polishing pads used for oxide CMP is that they become smoother with increasing polishing time, which degrades their slurry-retaining characteristics as well as their capability of mechanically wiping off the soft, chemically reacted layer from the oxide surface. This results in a decrease in removal rate. To counteract this smoothing tendency, polishing pads for oxide CMP typically undergo a conditioning process during or after wafer polishing.
Conditioning is generally performed by a rotating conditioning disk mounted on an arm above the polishing pad, as shown in FIG. 1. The mounting arm is generally computer controlled, and can sweep the conditioning disk across the polishing pad as the platen rotates. The sweep may be along an arch on the pad, or in the radial direction. By way of example, the APP-1000 Pad Conditioner built by Westech divides the polishing pad into ten segments along the radial direction. The rotation speed of the conditioning disk and the conditioning time at each segment can be programmed. The conditioning disk can be of various forms, including having embedded particles such as diamond. As the conditioning disk is rotated over the polishing pad, it roughens the pad surface. The amount of roughening, i.e., conditioning, at any location on the pad depends on parameters such as the conditioning time at that location, the downward force applied to the conditioning disk, the rotation speed of the conditioning disk, and the platen rotation speed. These parameters form the so-called conditioning recipe.
While conditioning the polishing pad can prevent the drop in removal rate by roughening the pad surface, removal rate reaches a saturation value at a certain conditioning time, known as the saturation conditioning time, and does not increase further with further conditioning. Furthermore, excess conditioning at any location causes pad thinning, thereby decreasing the polishing pressure and removal rate at that location. By way of example, an overconditioned central pad region can result in the dielectric polishing profile illustrated in FIG. 4. Overconditioning also adversely affects polishing pad lifetime due to cumulative pad thinning.
Generally, the conditioning recipe for the pads used in CMP production processes is a fixed process determined before polishing, according to empirically derived parameter values which are chosen to minimize WIWNU and maximize stability of average removal rate. Conditioning different regions of the polishing pad for different amounts of time is known as selective conditioning. Selective conditioning is facilitated by apparatus such as the aforementioned APP-1000 conditioner which allows user programming of the conditioning recipe at each pad segment. Selectively overconditioning pad regions which have higher removal rate can tailor the pad thickness profile and consequently the distribution of the strain field, to yield a more uniform removal rate across the wafer. An example of a fixed selective conditioning recipe is described by K. Acuthan et al in "Selective Conditioning and Pad Degradation Studies on Interlayer Dielectric Films", Proceedings, 1996 CMP-MIC Conference, ISMIC, 1996, pp 32-39.
A fixed conditioning process, however, even selective conditioning, cannot compensate for such factors as the aforementioned gradual and non-uniform changes in pad elasticity and thickness. Additionally, other factors may contribute to non-uniformity of removal rate, such as 1) manufacturing variations in polishing pad thickness and other pad physical and chemical properties, 2) carrier film wear, 3) conditioning wheel wear. These factors are also difficult to counteract with a fixed conditioning recipe. As a result, WIWNU of removal rate tends to increase as more wafers are polished. When the WIWNU exceeds specification, the polishing pad must be replaced.
A CMP apparatus which employed feedback of polishing data to progressively optimize the conditioning recipe would provide improved WIWNU, as well as increasing polishing pad lifetime.